This invention relates to a method of forming an article.
There are now many well known layer-wise manufacturing processes that take 3 dimensional computer aided design (3D CAD) files and create solid objects within the field known as ‘rapid prototyping’ and/or ‘rapid manufacture’.
In the ‘powder bed’ processes a point source of energy such as a laser or electron beam is selectively applied to a layer of powder as required on a layer by layer basis to build up a 3D part from a ‘slicing’ of a design file. Examples of this type of machine are the M270 produced by Electron Optical Systems GmbH (EOS) (laser “sintering”), the Realizer II marketed by MCP (laser “smelting”) and the Concept Laser from Hofmann Innovation Group AG (Laser “Cusing®”). These all take metal powders under an inert atmosphere and direct a laser onto a layer of powder to solidify. Similarly there are well known polymer and wax processing machines that either selectively apply material or selectively apply a means of binding/fusing/promoting or inhibiting to form a 3D solid object on a layer by layer basis.
A typical work flow from design to manufacture for these layer-wise processes is for a Computer Aided Design (CAD) file to be converted into an .stl file—a format created by 3D Systems a US company. Stl files describe only the surface geometry of a three dimensional object without any representation of color, texture or other common CAD model attributes. The stl file is viewed and verified, support structures (if required) formed and other manipulations carried out within a program such as ‘Magics’ a software from Materialise of Leuven Belgium. Then the file is converted into a ‘build’ file typically an .sli file being the .stl file “sliced” into e.g. 0.20 mm slices that are used to direct the laser for each physical layer of the build.
The .sli files are then loaded into the layer-wise processing machine where they are associated with a set of process parameters some of which, such as, laser power, scanning speed and offset can be associated with the individual .sli files (whilst other process parameters such as layer thickness are general to all .sli files) that go to make a batch of parts within a ‘job’.
The laser or other point source of heat has a finite spot size and creates a heat affected zone larger than its spot size and therefore to accurately build parts to the size required the centre of the point source of heat must be offset such that it is the edge of the heat affected zone that corresponds with the edge of the geometry to be formed. This ‘offset’ can be entered into software, however there is a practical problem. As it presently stands there is only a single global offset value for each .sli file. When forming e.g. small holes such as 0.5 mm in dia. the heat affected zone from the laser tends to close up the hole therefore a large offset value is required however this will affect the geometrical accuracy of the entire part.
More generally different aspects of a part may benefit from different build strategies.
It is well known that it is possible to build several distinct parts simultaneously and therefore multiple .sli files can be loaded into the laser sintering/melting machine to make a batch of parts each on a layer by layer basis.
From one aspect the invention is the splitting of a single part's file into multiple elements that represent different aspects or elements of that part, and either rejoining these files or loading the multiple .sli files into the layer wise manufacturing machine and applying different processing characteristics to the multiple elements' files where at least two files physically overlap in the layer-wise manufacturing process. By this means a certain feature such as a hole or screw thread may be built with one set of parameters such as ‘laser offset’ from one file whilst other aspects of that part may be built with different characteristics from another file. To achieve a satisfactory physical joining the two files preferably should overlap by at least one layer.
So for example a large structure containing tiny holes may be built effectively as two separate but overlying elements; one consisting of a large structure with larger holes and the other of rings that overlay the larger holes and form the precisely defined hole dimensions. The laser offset may be set in the one file to achieve critical dimensional accuracy at the large scale and the second file has the laser offset to achieve the small hole dimensional accuracy.
In another example a computer file representing a part with threaded studs may be split into an element without threaded studs and another file representing the array of threaded studs. The file of threaded studs may be manipulated e.g. laser offset to ensure their accurate formation and the file of the part without the studs manipulated to ensure its accurate formation. The two files are then overlapped sufficiently to ensure the studs are joined to the part and then the two files overlaid in the build file that is sent to the layer wise processing machine.
In another example a part may require a very good surface finish impossible to achieve with the state of the art in layer processing machines at certain areas. A machining or polishing allowance is selectively required and the part must therefore be oversize—but only in certain areas. This may be achieved by laser offset to make the part larger—however this is a global adjustment—effectively the laser is moved such that its centre point more nearly is at the outer edge of any geometry. This however would e.g. close up fine holes and in any case may not be required everywhere—only in the areas to be machined or polished. Again, by separating the one part into more than one element, manipulating these element files differently, creating a sufficient overlap as to joint them and then overlaying them with respect to one another in the build file sent to the machine is a good and useful method of achieving varying process characteristics within the single part made.
From another aspect the invention consists in a method of forming an article in a layer-wise manufacturing process from a computer software file representing the article including:                (i) dividing the file into sub-files in dependence of the size or other characteristics of features of the article to be created;        (ii) applying a process characteristic selected in dependence on the characteristic feature to each sub-file; and        (iii) manufacturing the article in accordance with the sub files.The sub files may be overlapped utilised to form features simultaneously.        
The process may include applying a point source of heat to a layer. The point of impingement on the layer being offsetable characterised in that the feature is the feature size and that the process characteristic is the value of the offset.
The feature to be optimised may be geometrical, such as a thread or orifice.
The sub files may be loaded into the layer wise manufacturing apparatus performing the process as separate files or, alternatively, the sub files may be aggregated into a single file before loaded into a layer wise manufacturing apparatus for performing the method. The sub files may further overlap in a layer by layer sense such that the process is performed on a feature more than once.
In any of the above cases the part may be formed from a metal or polymer powder sintered, melted or otherwise fused together to form a solid object by the application of a point source of heat such as a laser or electron beam.
The invention also includes an article formed by any of the methods set out above having first body with large holes from the first file and a number of annuli, created from the second file, located in space so as to fit within the large holes.
The invention further includes a method of forming an article in a manufacturing process from a computer software file representing the article including:                (i) creating a first sub-file representing the article and defining a number of openings having a dimension above a first predetermined value;        (ii) creating a second sub-file representing solid elements to lie within the openings to define final openings of second predetermined value less than the first predetermined value; and applying the two sub-files to manufacture the articles so that the body and the elements are formed integrally.        
Although the invention has been defined above it is to be understood it includes any inventive combination of the features set out above or in the following description.